Mycorrhizal associations are structures formed between symbiotic soil fungi and plant roots. Mycorrhizal fungi infect plant roots and enhance the plant's ability to take up water and nutrients, particularly phosphorus, from the soil. As obligate symbionts, mycorrhizal fungi are unable to live outside of a plant host.
It is estimated that more than 80% of flowering plant species on land are able to form mycorrhizal associations. Mycorrhizae fall into two groups based on the interacting plant hosts and fungal species. Woody Angiosperms and Gymnosperms interact with the fungi Basidiomycetes, Ascomycetes or Zygomycetes to form ectomycorrhizae. The Zygomycetes form endomycorrhizae with most other terrestrial plant species. Arbuscular mycorrhizae, an endomycorriza, are the most common form of mycorrhizae. Arbuscular mycorrhizal fungi interact nonspecifically with plants, and a single fungal species can symbiose with many plant species. (Gianinazzi-Pearson, V. The Plant Cell 8:1871-83 (1996); Harrison, M. J. Trends in Plant Science 2:54-60 (1997)). However, twenty percent of plants on land are unable to form or rarely form mycorrhizal associations, including the families Brassicaceae, Cyperaceae, Cruciferae, Chenopodiaceae, and Caryophyllaceae. (Raven, P. H., et al., BIOLOGY OF PLANTS. p.224, Worth Publishers, New York (1989); Sharma, S. et al. Microbiologia Sem. 13:427-436 (1997)).
Mycorrhizal associations provide a number of benefits to the host plant in addition to the well-documented enhancement of phosphate uptake. Mycorrhizal associations have been shown to stimulate uptake of nitrogen, zinc, copper, sulfur, potassium and calcium and to enhance the uptake of water. Mycorrhizal associations also protect the plant host from infection by pathogens. (Sharma, S. et al., Microbiologia Sem. 13:427-436 (1997)).
After infection with mycorrhizal fungi, crop plants exhibit improved growth. Economically important plants tested include vegetables, and field crops. See references in U.S. Pat. No. 5,096,481. Mycorrhizal fungal infection also enhances plant growth under stress conditions, including growth on reclaimed soils.
Previously, a root lectin, LNP (formerly called NBP46 or DB46) was isolated from young Dolichos biflorus root extracts. LNP is a 46 kDa protein that was isolated by affinity chromatography on hog gastric mucin blood group A+H substance conjugated to Sepharose (Quinn, J. M. and Etzler, M. E. Arch. Biochem. Biophys. 258:535-544 (1987)). The protein also has apyrase activity and appears to play an early role in rhizobium-legume biosynthesis (Etzler, M. E. et al., Proc. Natl. Acad. Sci. USA 96:5856-5861 (1999)). Genetic experiments indicate that the establishment of rhizobial symbiosis and mycorrhizal symbiosis share common steps (Albrecht, C. et al., EMBO J. 18:281-288 (1999)).
Identification of genes and proteins that modulate mycorrhizal fungal association with plants will further the beneficial use of mycorrhizal in agriculture. For example, plants that have enhanced mycorrhizal association will be able to use nutrients more efficiently and potentially require less fertilizer or be able to grow on less fertile soil. Plants that currently do not associate with mycorrhizal fungi could be transformed with genes to allow the association to take place, thereby increasing the range of environments for growth of these plants.
One of the obstacles to greater use of mycorrhizal fungi in agriculture is the difficulty in growing large quantities of the fungus. All mycorrhizal fungi are obligate symbionts and cannot be grown outside of the plant. Innoculants are most commonly made from the roots of infected plants. Enhancement of mycorrhizal fungal infection will improve the yield and efficacy of innoculant stocks. The present invention addresses these and other needs.